1. Field of the Invention
The present invention generally relates to a control apparatus for an internal combustion engine of an automobile or motor vehicle in which a bypass intake air passage is provided in parallel with a main intake pipe for affecting the idling operation of the engine. More particularly, the invention is concerned with a control apparatus which can positively assure a limp-homing operation (or backup operation) for driving a motor vehicle to a service station or the like place even when abnormality occurs in a main intake air flow control means provided in association with the main intake pipe. Further, the invention is concerned with a method and apparatus for ensuring controllability of engine speed as well as fuel injection and an automatic transmission even in the trouble-suffering state where the motor vehicle has to be driven by resorting to the limp-homing or backup function.
2. Description of the Related Art
In general, in the intake air control system for an internal combustion engine (hereinafter also referred to simply as the engine) of a motor vehicle, opening degree of a throttle valve disposed in an intake pipe (main intake air flow passage) is controlled via a wire-linkage which mechanically interconnects the throttle valve and an accelerator pedal to each other. Further, in an idling operation of the engine in which the main throttle valve is positioned to the fully closed state, the intake air flow is controlled by means of a bypass throttle valve installed in a bypass passage connected in parallel to the main intake pipe.
In recent years, however, there have been developed and increasingly adopted in practical applications the systems for electrically controlling the main throttle valve installed in the main intake pipe as well as the bypass throttle valve disposed in the bypass passage in an attempt to realize various functions in connection with operation of the motor vehicle, as is disclosed in, for example, Japanese Unexamined Patent Applications Nos. 294636/1991 (JP-A-H3294636) and 294630/1991 (JP-A-H3-294630).
For having better understanding of the present invention, it will he necessary to elucidate in some detail the background techniques as well as problems from which the control apparatuses known heretofore suffer.
FIG. 25 is a block diagram showing schematically a general arrangement of an engine air suction control apparatus known heretofore, wherein a traction control such as slip suppression is realized by controlling one of throttle devices disposed in series to each other.
Referring to FIG. 1, a main air flow passage (intake pipe) 1 for supplying air to an engine (not shown) as indicated by an arrow A is provided with a first air flow control means (hereinafter also referred to as the first throttle device) 11, the opening degree of which is controlled in dependence on extent or magnitude of actuation (depression stroke) of an accelerator pedal 4 in a manner described hereinafter and a second main air flow control means (hereinafter also referred to as the second throttle device) 12 which is installed in the main air flow passage (intake pipe) 1 in series to the first throttle device 11 at a position downstream thereof.
A bypass air flow passage 2 is connected in parallel to the main air flow passage 1 so that the air flow Gan bypass the first throttle device 11 and the second throttle device 12 through the bypass passage 2, wherein a bypass air flow control means (hereinafter also referred to the bypass throttle device) 21 is installed in the bypass passage 2 for the purpose of controlling rotation speed of the engine in an idling operation mode. Provided in association with the first throttle device 11 is a throttle position sensor 3 which serves for detecting an opening degree 1/4 of the throttle valve constituting a major part of the first throttle device 11.
Further, an accelerator pedal depression stroke sensor 5 is provided in association with the accelerator pedal 4 for detecting the depression depth or stroke of the accelerator pedal 4. An air conditioner 6 is shown as constituting an engine load, by way of example.
A control unit 7 which is in charge of overall control of the engine system outlined above and which may be constituted by an electronic information processor or microcomputer has inputs supplied with a variety of signals indicating the engine operation states inclusive of a signal D indicative of the depression stroke of the accelerator pedal 4. The control unit 7 serves to control the opening degree of the first throttle device 11 in dependence on the depression stroke or depth D of the accelerator pedal 4 in the normal operation mode of the engine. The opening degree 8 of the first throttle device 11 is detected by a throttle position sensor 3 to be fed back to the control unit 7. On the other hand, in the idling or bypass operation mode, the control unit 7 controls the bypass throttle device 21 so that the throttle valve thereof is set to a position corresponding to a desired value of the opening degree. Besides, the control unit 7 is in charge of control of various engine loads, one of which is represented by the air conditioner 6.
Although not shown in FIG. 25, the control unit 7 incorporates therein a traction control module or means for reducing the engine output torque in order to prevent slippage of the motor vehicle upon starting operation thereof by closing the throttle valve of the second throttle device 12. Additionally, the control unit 7 incorporates a diagnosis module or means (not shown) for diagnosing the first throttle device 11 as to occurrence of trouble or abnormality therein. If occurrence of abnormality in the first throttle device 11 is decided by the diagnosis means, the second throttle device 12 is so controlled as to close the associated throttle valve.
FIG. 26 is a block diagram showing in more concrete a structure of a major part of the engine system shown in FIG. 1 partially in section. In FIG. 26, reference numerals 1 to 7, 11, 12 and 21 denote parts same as or equivalent to those designated by like reference numerals in FIG. 25, respectively.
Referring to FIG. 26, the first throttle device 11 installed in the main air flow passage 1 is comprised of a throttle valve 11a and an actuator, e.g. an electric motor 11b for operating the throttle valve 11a. Similarly, the second throttle device 12 includes a throttle valve 12a and an actuator, e.g. an electric motor 12b for operating the throttle valve 12a.
On the other hand, the bypass throttle device 21 is comprised of a linear solenoid valve 21a and an electromagnetic actuator 21b for driving the valve 21a. The electromagnetic actuator 21b is adapted to be periodically excited by the control unit 7 for holding stationarily the linear solenoid valve 21a at a given position.
An air cleaner 13 is mounted at the entrance of the main air flow passage (main intake pipe) 1. Further, an air flow sensor 14 is installed in the main air flow passage 1 at a position downstream of the air cleaner 13 for detecting an intake air flow rate B. Provided in association with the first throttle device 11 is an idle switch 15 for detecting the fully closed state of the throttle valve 11a of the first throttle device 11. Further, installed in the main air flow passage 1 at a position downstream of the second throttle device 12 is a fuel injector 16 for injecting a fuel into the main air flow passage 1. An intake manifold 17 is formed in fluidal communication with the main air flow passage 1 at a location downstream of the fuel injector 16 for accommodating therein the air-fuel mixture which is supplied to cylinders (only one cylinder 18 is shown) of the engine. A water temperature sensor 19 is installed in association with the engine cylinder for detecting the temperature T of cooling water W for the cylinder 18 (or radiator thereof).
The air flow rate signal B generated by the air flow sensor 14, the fully closed throttle signal outputted from the idle switch 15 and the temperature signal T from the water temperature sensor 19 are inputted to the control unit 7 similarly to the signals indicating other engine operation state parameters (not shown).
In the foregoing description made by reference to FIGS. 25 and 26, it has been assumed that the first throttle valve 11a is controlled by the electromagnetic actuator 11b. It should however be understood that the first throttle valve 11a may be controlled straightforwardly by the accelerator pedal 4 through a linkage means. FIG. 27 shows schematically another exemplary structure of the first throttle device 11 and that of the second throttle device 12, wherein the throttle valve 11a and the accelerator pedal 4 are directly linked to each other by means of a wire 11c, while the second throttle valve 12a is adapted to be controlled by an electric motor. For more details of the illustrated structure of the first throttle device 11 and the second throttle device 12 shown in FIG. 27, reference should be made to, for example, Japanese Unexamined Patent Application Publication No. 294636/1991 (JP-A-H3-294636).
In the case of the engine system shown in FIGS. 25 and 26, the main air flow control means is constituted by the first throttle device 11 and the second throttle device 12 disposed in series to each other. However, the main air flow control means may be implemented by a single electrically driven throttle device 11, as shown in FIG. 28. More specifically, referring to FIG. 28, a throttle valve 11a adapted to be driven by an electric motor 11b is continuously controlled so as to serve for the function of the second throttle device 12 shown in FIG. 27 as well. Thus, the second throttle device 12 can be spared. The structure of this throttle device 11 is disclosed in detail, for example, in JP-A-H3-294630.
FIGS. 29A and 29B are characteristic diagrams illustrating relations between a desired or target throttle opening degree .theta..sub.0 and the depression depth or stroke D of the accelerator pedal in the form of functions F.sub.1 (D). More specifically, FIG. 29A shows a non-linear relation in which the target throttle opening degree .theta..sub.0 is so set that it increases more steeply within a predetermined range of the accelerator pedal stroke D which is set close to the upper limit thereof, while FIG. 29B shows a relation in which the target throttle opening degree .theta..sub.0 is set essentially as a linear function of the accelerator pedal stroke D.
Next, referring to FIGS. 25 to 29A and 29B, description will turn to operation of the conventional engine intake air control apparatus of the structure described above. The throttle valve 12a of the second throttle device 12 driven under the control of the control unit 7 is susceptible to the control only in the closing direction and ordinarily remains in the open state. Referring to FIGS. 25 to 27, the intake air flow rate B and hence the engine rotation speed (rpm) as well as the engine output torque are controlled in dependence on the throttle opening degree .theta. of the throttle valve 11a of the first throttle device 11.
Upon activation of traction control, the control unit 7 suppresses the engine output torque by decreasing the intake air flow rate B in order to prevent the driving wheels of the motor vehicle from slippage. By way of example, upon starting of the motor vehicle, the second throttle device 12 is controlled so that the throttle valve 12a is closed to thereby reduce the flow rate B of the intake air A (and thus the engine output torque) in order to prevent the slippage of tires of the wheels. This control is referred to as the traction control.
In the ordinary running state of the motor vehicle, the control unit 7 determines the desired or target value .theta..sub.0 of the throttle opening degree .theta.of the throttle valve 11a in dependence on the accelerator pedal depression stroke D in accordance with the relation illustrated in FIG. 29A or 29B to thereby control the electric motor 11b so that the throttle opening degree signal .theta. derived from the output of the throttle position sensor 3 coincides with the target value .theta..sub.0. At that time, the throttle valve 12a of the second throttle device 12 is maintained in the open state.
The relation between the target throttle opening degree .theta..sub.0 and the accelerator pedal depression stroke D illustrated in FIG. 29B applies valid to the control in the case where the throttle valve 11a is mechanically coupled to the accelerator pedal 4, while in the case of the control corresponding to the relation illustrated in FIG. 29A, drivability of the motor vehicle can be changed or modified as desired.
The control unit 7 arithmetically determines or calculates the amount of fuel to be injected to the engine on the basis of the intake air flow rate B measured by the air flow sensor 14 and the engine speed (rpm), whereon the amount of fuel as calculated is corrected by taking into account the other operation parameters to thereby generate a signal indicative of the optimal amount of fuel injection for controlling the operation of the fuel injector 16 correspondingly.
In this manner, a demanded amount of the intake air A is fed to the engine through the main air flow passage 1 via the first throttle device 11 which is controlled by the electric motor 11b so as to provide the desired opening degree .theta. and the second throttle device 12 whose throttle valve 12a is constantly maintained in the open state. The injector 16 charges an amount of fuel which corresponds to the intake air flow B at a predetermined timing to produce an air-fuel mixture which is then charged to the engine cylinder 18 and undergoes explosive combustion at a predetermined ignition timing. Ultimately, the engine generates an output torque which corresponds to the depression stroke D of the accelerator pedal 4 (and thus corresponds to the intake air flow rate B).
In a steady running state of the motor vehicle at a high engine rotation speed (rpm), the control unit 7-holds the throttle valve 12a of the second throttle device 12 in the open state so long as the first throttle device 11 operates normally, while controlling constantly the flow rate B of the intake air A flowing through the main air flow passage 1 by the first throttle valve in dependence on the accelerator pedal depression stroke D.
Further, in the idling operation in which the throttle valve 11a is fully closed, the control unit 7 triggers the idling engine speed control at a low speed in response to the signal generated by the idle switch 15 when the throttle valve 11a is fully closed or in response to disappearance of the signal D indicating the actuation of the accelerator. More specifically, the control unit 7 controls the linear solenoid valve 21a of the bypass throttle device 21 (FIG. 26) to thereby maintain the engine rotation speed (rpm) at a target value (e.g. 700 rpm) for the idling operation.
On the other hand, in the steady operation state, the diagnosis means incorporated in the control unit 7 checks the signal indicative of the throttle opening degree .theta. of the throttle valve 11a as fed back from the throttle position sensor 3 and decides that the first throttle device 11 operates normally so long as a demanded opening degree thereof is detected for the effective depression stroke D of the accelerator pedal 4. If otherwise, the diagnosis means decides that the first throttle device 11 suffers abnormality. Further, occurrence of abnormality may be decided when the throttle opening degree 8 does not vary even when the electric motor 11b is electrically energized.
When abnormality of the first throttle device 11 is decided, the control unit 7 closes the throttle valve 12a of the second throttle device 12 to thereby stop the engine in order to prevent occurrence of unwanted situation such as overrun of the motor vehicle.
Needless to say, when the second throttle device 12 falls in a fault in the fully closed state, the engine operation is forced to stop.
In the intake system where only one throttle valve 11a is employed, as is illustrated in FIG. 28, the throttle valve 11a regulates the flow rate B of the intake air A under the control of the control unit 7 by the motor 11b. However, upon occurrence of a fault in the throttle valve 11a, the engine will be stopped when the fault takes place in the fully closed state of the throttle valve 11a, while the engine may overrun when the fault occurs in the fully opened state of the throttle valve 11a.
For the reason mentioned above, the diagnosis means of the control unit 7 decides that the throttle valve 11a suffers abnormality when the demanded intake air flow rate B corresponding to the depression stroke D of the accelerator pedal 4 is not detected, whereby the throttle valve 11a is forcively closed, which of course results in that the engine is forcively stopped.
As is apparent from the foregoing description, occurrence of abnormality in the first throttle device 11 necessarily leads to the stoppage of the engine. In other words, it is impossible to ensure a minimum running capability (i.e., a so-called limp-homing or backup operation) which allows the driver to run the motor vehicle in any way to a destination such as a service factory or the like place.
At this juncture, it should be mentioned that the bypass throttle device 21 shown in FIG. 26 is not in the position to allow the engine to generate an output torque of magnitude enough to ensure the limp-homing or backup function because the air flow fed through the bypass throttle device 21 is excessively small. At any rate, it is impossible to drive the motor vehicle by controlling the intake air through the bypass throttle device 21 in the control apparatus described above by reference to FIGS. 25 to 29.
Besides, it is to be added that imparting of such air flow rate control capability to the bypass throttle device 21 as to allow the motor vehicle to be driven is undesirable from the practical standpoint in view of the possibility of overrun of the motor vehicle upon occurrence of a fault in the bypass throttle device 21 in the fully opened state thereof. Of course, an additional intake air control means may be resorted to for ensuring the limp-homing (backup) operation mode, which will however involve very high expensiveness, to a disadvantage from the economical standpoint.
As is now apparent, the engine intake air control apparatus known heretofore in which only one bypass passage 2 is provided in parallel to the main air flow passage 1 and in which the air flow susceptible to the control of the bypass throttle device 21 installed in the bypass passage 2 is set to the flow rate for the idling operation is incapable of realizing the above-mentioned limp-homing (backup) function when a fault takes place in the main air flow control means 11 or 12 installed in association with the main air flow passage (intake pipe) 1.
It is also known from, for example, Japanese Unexamined Patent Application Publication No. 8441/1986 (JP-A-61-8441) to regulate or control the intake air flow fed to the engine by means of a throttle valve installed in the intake pipe, wherein the throttle valve is so controlled as to be set to the fully closed state when abnormality is detected in the devices provided in associated with the throttle valve, while allowing the intake air flow to be adjusted by means of an auxiliary control system which is adapted to control the intake air flowing through a bypass passage so that the motor vehicle can be operated notwithstanding of occurrence of the abnormality mentioned above.
FIG. 30 shows a structure of the control apparatus adapted for controlling the intake air flow of an engine, which is disclosed in the publication mentioned above.
Referring to FIG. 30, the control apparatus includes a main control system, an auxiliary control system and an abnormality detecting system.
The main control system is comprised of an accelerator pedal depression stroke sensor 102 for detecting the depression stroke (i.e., magnitude of actuation) of an accelerator pedal 101, a main throttle valve 110 disposed in an intake pipe 115 which leads to the engine, a throttle position sensor 108 for detecting the position (opening degree) of the main throttle valve 110, a control signal generating means 111 for arithmetically determining or calculating a quantity for driving the main throttle valve 110 on the basis of the accelerator pedal depression stroke detected by the accelerator pedal depression stroke sensor 102 and an actual throttle valve position detected by the throttle position sensor 108, and a main throttle valve driving device 106 for driving the main throttle valve 110 in response to a signal indicative of the quantity for driving the main throttle valve 110 as calculated by the control signal generating means 111.
The abnormality detecting system includes an abnormality detecting means for detecting the occurrence of abnormality in the main air flow control means and/or the devices provided in association therewith.
The occurrence of abnormality in the main air flow control means as well as the devices associated therewith may be determined by deciding, for example, whether difference between a desired or target opening degree of the main throttle valve 110 and an actual opening degree thereof exceeds a predetermined value, or whether the detected value of the throttle position sensor 108 is deviated from a predetermined upper or lower limit value, or whether a detected value of the position of the main throttle Valve 110 is abnormal by comparing signals outputted by a plurality of the throttle position sensors 108 when provided, or whether an estimated value of the throttle valve opening degree as determined on the basis of signals outputted from an intake air flow sensor (not shown) and an engine speed (rpm) sensor 103 instead of the output signal from the throttle position sensor 108 indicates abnormality, or whether the throttle valve opening degree remain unchanged notwithstanding of change in the control signal generated by the control signal generating means 111, or whether breakage or short-circuit occurs in the electrical connection for the main throttle valve driving device 106.
When abnormality is detected by the abnormality detecting means 112 as a result of execution of any one of the abnormality detecting procedures enumerated above, a throttle valve closing means 107 is activated to thereby set the throttle valve 110 to the fully closed state.
On the other hand, the auxiliary control system is comprised of the accelerator pedal depression stroke sensor 102, a bypass throttle valve device 109 disposed in an bypass passage 116 for allowing the intake air to flow by bypassing the main throttle valve 110, a bypass throttle valve control means 113 for calculating the quantity for driving the bypass throttle valve 109 in dependence on the depression stroke of the accelerator pedal 101 detected by the accelerator pedal depression stroke sensor 102, and a bypass throttle valve driving means 114 for driving the bypass throttle valve means 109 in response to reception of the control signal from the bypass throttle valve control means 113.
Parenthetically, the control signal generating means 111, the abnormality detecting means 112 and the bypass throttle valve control means 113 are incorporated in the form of modules in a signal processing unit 105 (engine controller) which may be realized by using a microcomputer.
It should further be mentioned that the bypass throttle valve means can be used as an intake air flow control means in an idling operation mode of the engine. In that case, the bypass throttle valve control means 113 is supplied with the engine speed (rpm) signal derived from the output of the engine speed sensor 103 and a signal derived from the output of an engine load sensor 104 typified by the signal indicative of operating state of an air conditioner, to thereby effect the idling speed control on the basis of these detected quantities.
Next, description will be made of operation of the control apparatus by reference to FIG. 31.
First, in a step S101, an accelerator pedal depression stroke is detected by the accelerator pedal depression stroke sensor 102 to thereby calculate or arithmetically determine a target throttle opening degree on the basis of the output from the accelerator pedal depression stroke sensor 102.
In a step S102, an actual opening degree of the main throttle valve 110 is detected by the throttle position sensor 108 which constitutes a part of the main control system.
When abnormality is found in the detection of the throttle valve position as effected in the step S102, the abnormality is detected by the abnormality detecting means 112 in a step S103, which is then followed by the execution of a step S107 in which the main throttle valve 110 is fully closed by the throttle valve closing means 107 to thereby inhibit the intake air flow control of the main control system. At the same time, the intake air flow control is transferred to the auxiliary control system in a step S108 where the opening degree of the bypass throttle valve 109 is controlled or regulated on the basis of the output of the accelerator pedal depression stroke sensor 102 to thereby guarantee at least the minimum capability of driving the motor vehicle to a service station or home even in the state where the vehicle suffers from abnormality in the main control system (i.e., the limp-homing or backup capability).
On the other hand, when the aforementioned decision step S106 results in negation (NO), a quantity for driving the main throttle valve 110 is calculated by the control signal generating means 111 in a step S104, and the main throttle valve 110 is driven by the main throttle valve driving device 106 in accordance with the quantity as determined (step S105). When abnormality is detected in the course of driving the main throttle valve 110 in the step S106, the steps S107 and S108 are executed, whereby the intake air flow control is transferred to the auxiliary control system.
In the apparatus described above, it is noted that the amount of fuel injected into the engine is controlled on the basis of the intake air flow and the engine rotation speed (rpm), wherein similar fuel injection control procedure is performed in both the cases where the intake air flow is controlled by the main control system and where the intake air flow control is effected by the auxiliary control system. As a result of this, there arises problems mentioned below.
Heretofore, the bypass throttle valve driving means 114 constituting a part of the auxiliary control system is implemented by using an inexpensive actuator of a low control speed such as a stepping motor, a DC motor, a DC solenoid or the like. Accordingly, an appreciable delay is involved in the response of the bypass throttle valve driving means 114 to a demand for rapid reduction of the intake air flow which is issued when release of the accelerator pedal is accompanied with manipulation of the clutch. In this case, the engine speed (rpm) may increase steeply, to a problem.
By way of example, let's assume that the driver changes the gear ratio from a first to a second stage in the course of driving the motor vehicle with the intake air control being effected by the main throttle valve control system. In that case, the driver will have to release the accelerator pedal simultaneously with the manipulation of the clutch in order to prevent the engine speed from increasing steeply due to the gear ratio change. However, when the intake air flow control is being performed by the auxiliary control system (bypass intake air control system) instead of the main control system because of abnormality thereof, the release of the accelerator pedal simultaneously with the upshift of the gear ratio is reflected onto the intake air flow control only with a significant time delay particularly when the bypass throttle actuator of low response characteristic as mentioned above is employed as the bypass throttle valve driving means 114. In that case, the engine speed will increase rapidly because of a significant amount of intake air fed to the engine before the bypass throttle valve 109 is driven in the closing direction, to uncomfortableness.
Certainly, there is known a control apparatus for an internal combustion engine, which apparatus is designed to ensure a backup operation such as limp-homing (backup) operation mode even in the case where a fault occurs in the throttle valve control system in the fully closed state of the throttle valve, as is disclosed in Japanese Unexamined Patent Application No. 286837/1990 (JP-A-H2-286837) or No. 8441/1986 (JP-A-61-8441). However, this known apparatus still suffers a problem remaining to be solved, which will be made clear in the following.
FIG. 32 is a schematic block diagram showing a general arrangement of a simplified example of such a control apparatus as mentioned above. Referring to the figure, an internal combustion engine 1 for a motor vehicle is provided with an intake pipe 203 in which a throttle valve 202 is disposed for regulating or controlling the amount of air to be charged in the engine. The throttle valve 202 is adapted to be driven by a throttle actuator 204 which may be constituted by a stepping motor, a DC motor or the like. The throttle valve 202 is operatively coupled to the throttle actuator 204 by means of a shaft 205. A return spring 206 is disposed around the shaft 205 for forcively setting the throttle valve 202 to the closed state upon occurrence of abnormality in the throttle control system.
Further provided in association with the throttle valve 202 is a throttle position sensor (TPS) 207 which serves for detecting the opening degree of the throttle valve 202. A bypass passage 208 is connected to the intake pipe 203 in parallel for allowing the intake air flow to bypass the throttle valve 202. A bypass control valve 209 is installed in the bypass passage 208 primarily for the purpose of controlling the auxiliary intake air flow through the bypass passage 208 in the idling operation of the engine. An accelerator pedal 210 of the motor vehicle is equipped with an accelerator pedal depression stroke sensor (or accelerator pedal position sensor or APS) 211 for detecting the depression or actuation stroke of the accelerator pedal 210. There are further provided an engine speed (rpm) sensor 212 for detecting the engine speed, i.e., rotation number thereof and a load sensor 213 for detecting a load of equipment such as a power steering system, an air conditioner and so forth. A signal processing unit 214 which may be constituted by a microcomputer is in charge of controlling the throttle actuator 204 and the bypass control valve 209 on the basis of the signals generated by the various sensors.
The engine 201 includes a speed change gear (including a solenoid valve (SV) or the like) for controlling an automatic transmission 216 by utilizing the output of the throttle position sensor 208 as the engine output torque information, a fuel injection control means 217 for controlling the amount of fuel to be injected into the engine through a fuel injector 218 in dependence on the opening degree information available from the output of the throttle position sensor 208.
Next, operation of the control apparatus will be described by reference to a flow chart of FIG. 33. Parenthetically, it should be mentioned that arithmetic operations or calculations as well as decisions mentioned below are executed by the signal processing unit 214 which may be constituted by a microcomputer or the like.
In a step S201, a difference .beta. is determined between an accelerator pedal stroke .alpha. indicated by the output signal of the accelerator pedal depression stroke sensor 211 interlocked with the accelerator pedal 210 and an actual throttle opening degree .theta..sub.r indicated by the output signal of the throttle position sensor 208.
The relation between the accelerator pedal stroke .alpha. and the actual throttle opening degree .theta..sub.r is given by a predetermined function such that the actual throttle opening degree .theta..sub.r progressively increases as the accelerator pedal stroke .alpha. increases until the throttle valve 202 is fully opened. Accordingly, so long as the throttle valve control system is normal, the difference .beta. must not exceed a predetermined valve .beta..sub.1.
Accordingly, in a step S202, decision is made as to whether or not the difference .beta. is greater than .beta..sub.1. If so, it is determined that the throttle valve control system suffers some trouble, and the throttle actuator 204 is then deenergized so that the throttle valve 2 assumes the fully closed state in a step S203. At the same time, the bypass control valve 209 is driven in such direction that the auxiliary or bypass intake air flow can increase (step S204). In this manner, the backup operation (limp-homing (backup) operation mode) can be assured notwithstanding of occurrence of trouble in the fully closed state of the throttle valve 202.
On the other hand, when it is decided in the step S202 that the difference .beta. is smaller than the predetermined value .beta..sub.1, indicating that the throttle valve control system operates normally, a normal control is performed in a step S205 in which the opening degree of the throttle valve 202 is controlled by driving the throttle actuator 204 in dependence on the output signal of the accelerator pedal depression stroke sensor 211. Of course, the idling engine speed control can be carried out by controlling the bypass control valve 209 (step S206).
The control apparatus described above however suffers a problem that the vehicle speed can not arbitrarily be set at a desired value in the backup (limp-homing) operation for driving the motor vehicle having trouble in the throttle valve control system to a service station. More specifically, in the backup operation mode, the bypass control valve can be driven only in the direction to increase the auxiliary intake air flow, making thus it impossible to set the vehicle speed at a desired value.
Moreover, because the main throttle valve 202 is constantly maintained in the fully closed state when trouble takes place in the throttle valve control system, performances of other control means such as the speed change gear control means 215 and the fuel injection control means 217 which are conventionally designed to operate on the basis of the output of the throttle position sensor 208 will be degraded, incurring degradation in the drivability of the motor vehicle in the backup (limp-homing) operation mode such as slow response to actuation of the accelerator pedal or change of the gear ratio.